It's not data sheet nonsense. A 3.3V part will have min/max signal level and it depends on a number of factors including process/technology (Bipolar, JFET, CMOS) at the inputs as well as the designers/manufactures choice of how to balance certain specs. Approaching the rails of the analog section will no doubt increase distortion and it depends on the process/tech used and whether or not rail-to-rail topologies for any one process/tech are incorporated.
1Vrms is very common and it's due to the fact that 1Vrms is approximately 2.8 V pk-pk which is well within the capabilities of 3.3V powered analog front ends of ADC's while being able to spec a low distortion and actually leverage the bit depth of the ADC (24-bits is a lot of signal precision). No, the WM8371 or AK4556 will not properly resolve a signal with 3.3V peak-peak. That single will get clipped. There is no data sheet nonsense ... follow the data sheet :-)
The WM8731 part may *sound* good to you but I suspect that with your ears and setup other parts will sound good too. The data sheets have figures not to be overlooked and the dynamic range, SNR, distortion, and SNR+distortion measurements are standard and shared (with variations noted in the data sheet) across manufacturers. The AK4556 is spec'd better.
Do you need a PGA controlled by I2C/SPI or hardware pins? Features are great if you intend to use them thoughtfully. If not then just keep it simple. 1Vrms is a good start for resolving signals from guitar pickups. You'll not clip with hum buckers and single-coils and even most actives, and you only need a unity gain buffer for the high impedance input. Easy.
No one making real effects uses the WM8731, or the AK4556 for that matter (they're old and there are way better parts now). But if you're not picky and just getting started keep it simple until you actually need to make it *more* complicated. The AK4556 is great for that. But hey I don't have a vested interest here - just speaking from experience and opinion.
Want more dynamic range on a smaller guitar signal? Yeah the PGA *seems* like a good idea but is it in practice? Not for me and not for modern digital effects products since the analog front ends are clean and the ADC is spec'd well. I have tons of digital gain on signals coming from a low noise TI ADC (PCM4201) with a unity gain buffer and I don't clip, the signal is remarkably clean even after gain to the point of (after measuring) not believing that it could be that good. No PGA, just a low-noise layout and good parts.
Concerned about pickup type, dynamic range, and noise performance? Well you already have to add an opamp to buffer your guitar signal. So just use a dual op amp, buffer first stage, attenuate or amplify next state, controlled with a pot. If your front end is clean and the ADC is decent you won't care about variable input gain.
Take a listen to this ... no PGA and fixed 1Vrms input range. Various guitars used and various effects. And the ADC and analog front end of this version of the pedal isn't very good. The user doesn't have to mess with input gain.
https://vimeo.com/336877986https://vimeo.com/348616450/1a28894c1b